A comic response you might get from a glass contractor if you ask that question is: “Whack it with a hammer. If it shatters in hundreds of tiny pieces, then it was safety glass!” Actually, that would identify it as tempered glass, which is one type of safety glass. The other type is laminated glass, which is sometimes used for residences, but more often seen in car windshields and storefront glass. It breaks in a characteristic spider web pattern that marks the location of the windshield impacts in a car accident, because the center layer of clear vinyl keeps the glass shards intact.

TEMPERED GLASS

    Tempered glass must have a small label etched in a corner of the glass, officially known as the “glass monogram,” but called a “bug” in the construction trades. It is required to identify the glass as tempered, have the manufacturer name, and the ANSI and CPSC standards under which it was manufactured. The bug may also include an SGCC approval number, which is a voluntary industry certification program.

   When you are looking for the bug, remember that it will be in a corner of the glass, very small, and barely legible because it is a light etching of the glass surface. When a door or window has multiple small glass panes, at least one pane must have the full bug, and the others only need to have a “16 CFR 1201” marking. In some cases, the bug will be partially or completely concealed by the frame. 

    Glass is tempered during the manufacturing process by rapidly cooling the outer surfaces with chilled air, but leaving the inner core still viscous. When the glass has completely cooled, the core is in tension and outer layers are in compression. This makes it four times more resistant to impact than regular (annealed) glass and, when shattered, it fractures perpendicular to the face of the glass into small pebble-size pieces.

LAMINATED GLASS 

    Laminated glass is more difficult to identify. It may not have a bug, because most building codes don’t require it and, If there is a bug, it may reference a DOT (Department of Transportation) approval code. Also, laminated glass can be cut to size after manufacturing, and the bug may have been cut off. Tempered glass will shatter if cut or drilled after manufacturing, and must be tempered to the exact size required, so the bug is there somewhere. 

    Industry professionals identify unmarked laminated glass by the multiple reflections visible when you put an object next to the glass. Glass that is not laminated shows only two reflections from the two surfaces of the glass. 

     A federal law mandates safety glass for areas that have the possibility of impact by a person, such as sliding glass doors and low windows in walking areas. The stained glass panel at foot level at a stair landing, shown below, is an example of a window that really should be safety glass, but is acceptable by code because it is considered “decorative glazing." Pretty, but very dangerous.

     The law did not go into effect until July 6th, 1977, so earlier windows and doors may not be safety glass. Older sliding glass doors are considered so dangerous by some building departments, such as Los Angeles, that they are required to be replaced or protected with safety film when the property is sold. 

    ScotchShield®, manufactured by 3M, is one of several brands of safety film that can be applied to old doors and windows to provide shatter protection. An installer typically adds a small label indicating that the glass now meets the CPSC standard for safety glass after completing the work.

    Also see our blog posts Frequently Asked Questions (FAQ) about Safety Tempered Glass and Where is safety/tempered glass required for windows and doors? 

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  To learn more about doors and windows, see these other blog posts:

• What causes sweating (condensation) on the inside of windows in the winter? 

• Is every exterior door of a house required to have a landing outside? 

• What are the small slots at the bottom of the outside of my window? 

• Why does condensation form on the outside of some windows and not others in the morning? 

• Why is the garage door track a white tube? 

• What is the raised metal plate on the floor under the garage door?

• Why do I have to hold down the button to close the garage door? 

• How can I tell if a window or glass door is safety glass? 

• What are the code requirements for safety tempered glass for doors?• 

• What are the building code requirements for a door from the garage to the house?

• What is "low-E" window glass? 

• What does ANSI 297.1 on glass mean? 

• What is an egress window?

• Does a home inspector test all the windows and doors in a home? 

• How difficult is it to change a window to french doors or a sliding glass door?

• How do you determine if a door is left-handed or right-handed?

• Why are window security bars dangerous? 

• What are the common problems you find inspecting windows?

• What is causing a foggy haze on my windows? 

• What do those numbers on the manufacturer's stickers in new windows mean?

• What does a home inspector check on an electric garage door? 

• What is the tempered label on glass at windows and sliding glass doors called?

• Do I need to have two exterior exit doors in my house? 

• When is safety glass required for windows at stairs and stair landings?

   Visit our DOORS AND WINDOWS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.